An axial-piston pump or motor, i.e. an axial-piston hydrostatic machine, generally comprises a cylinder drum rotatable about an axis and formed with a plurality of angularly equispaced cylinder bores, each of which receives slidably a piston having a ball head external of the cylinder drum and bearing upon a reaction surface which may be tilted to control the displacement of the machine. The reaction surface, e.g. a swashplate, may be engaged by a respective slide shoe which has a socket receiving the ball head of the respective piston.
In swashplate-type machines of this kind, it is a common practice to provide a hydraulic-medium cushion within the guide shoe which is maintained under pressure by a passage traversing the piston and communicating, at the working end thereof, with the compartment or chamber of the cylinder bore which receives the hydraulic fluid from or delivers the hydraulic fluid to a valve plate or control surface upon which the cylinder rides.
The purpose of this cushion is to provide a lubricating film between the shoe and the reaction surface along which the shoe slides and, in addition, to maintain a floating relationship between this shoe and the reaction surface. The fluid delivered to the cushion also lubricates the juxtaposed surfaces of the piston head and its socket.
In conventional machines of the aforedescribed type, the pressure in the pressure cushion corresponds to the pressure at the working-end face of the piston and can correspond hydrostatically to the force which is applied by the piston to the reaction surface.
To this end, the cushion is connected to the working chamber ahead of the working end face of the piston.
In order to reduce the inertia of the cylinder drum and piston assembly which is rotated about the axis of the cylinder drum, the piston may be made hollow and can be provided with a post connecting the ends of the piston and provided with a passage which delivers the hydraulic medium from the working end of the piston to the bore provided in the ball head thereof.
It is important to prevent an excess flow of fluid to the pressure cushion and, therefore, a lifting of the slide shoe away from the reaction surface with escape of fluid to an excessive degree. To this end, the passage between the working chamber of the cylinder and the fluid cushion is provided with a throttle which ensures delivery of the pressurized fluid to the fluid cushion but prevents an excessively rapid flow thereto.
With such a throttle constriction, however, there arises the danger that impurities or contaminants entrained in the fluid medium will plug the constriction and prevent further flow of the pressure medium to the cushion. As a result, the slide shoe may come to rest around its periphery with the full piston force against the reaction surface and result in excessive wear of the shoe and the reaction surface upon which the shoe rides.
It should be mentioned that pistons having fine filters are already known and these filters serve to trap the impurities and prevent them from entering the pressure cushion. When such contaminants are allowed to pass into the pressure cushion, they can pass between the edge of the slide shoe and the reaction surface upon which the latter rides and cause wear of these juxtaposed faces.
Such filters, however, must be designed so that their useful life corresponds to the useful life of the entire axial-piston machine since a replacement of the filters during the normal life span of the machine cannot be done effectively. In other words, in order to make the machine as maintenance-free as possible, the filters cannot be so positioned and designed that they are replaceable.
These requirements create new difficulties. Firstly, the filters must be relatively large and of complicated construction to ensure that they will not become plugged by the contaminants which may be carried by the fluid medium. In fact, because of the complexities involved in designing and constructing pistons having fine filters therein, it has been found to be essential to carry out a fine filtering of the hydraulic medium before it enters the transmission or machine so as to minimize the contaminant pickup by filters in the pistons.
Naturally, even this expedient does not prevent large particles from penetrating into the pistons and such particles may block a substantial portion of the flow cross section of the piston passage. If any such particles should happen to reach the throttle, the disadvantages previously mentioned arise.
Even a fine filtering of the fluid medium does not exclude this possibility because, in many instances, the contaminants arise from wear of particles on the seals of the machine, e.g. the sealing rings or O-rings which are commonly used therein.
Sieve-type filters have been provided in pistons as well but have been found to be ineffective under the acceleration forces which operate upon the pistons. The mechanical characteristics of sieve filters also pose difficulties if long useful lives are to be ensured.